Numerous methods and devices for determining the flow properties of fluid media, that is, liquids or gases, are known from the related art. The flow properties may basically be any physically and/or chemically measurable properties which qualify or quantify a flow of the fluid medium. In particular, this may be about the relative or absolute moisture content of a gas or a gas mixture. As is known, moisture sensors find use in the intake tract or the exhaust tract of internal combustion engines. In this case, it is important for regulating the combustion efficiency, for example, particularly for a combustion that is without residue and clean, to ascertain, among other things, the moisture content of the fresh air supplied, in order to enable setting the appropriately desired mixture ratio of fresh air and recirculated exhaust gas and subsequently to determine a quantity of fuel for a predetermined mixing in the cylinder of the internal combustion engine. The information on the moisture content of the environmental air supplied, as well as of the exhaust gas is used, however, by the engine control unit for a plurality of regulated processes.
Such sensor devices for use in the intake tract or the exhaust tract of internal combustion engines are known, for instance, from German documents DE 10 2010 043 062 A1 or DE 10 2010 043 083 A1. The sensor devices provided according to those documents provide a combination of a plurality of sensors, in order to register a plurality of flow properties of fluid media. In particular, one is able to gather from DE 10 2010 043 083 A1, with respect to the ascertainment of the moisture content of the fluid medium, that, in the sensor device provided there, a moisture sensor or a moisture module is taken up which, together with the sensor device, is inserted into a flow channel of a fluid medium. Within the flow channel, the entire fluid medium flows past the semipermeable membrane of the moisture sensor. In this instance, drops of condensed water occurring at the edge of the inlet opening, situated around the membrane, may get stuck there and subsequently wet the semipermeable membrane. The permeability of the membrane may be impaired, in this instance, in such a way that a reliable ascertainment of the moisture content of the medium is no longer ensured. Similarly, because of the direct action on the membrane by the fluid medium, which is usually an unfiltered and/or mechanically compressed process gas, the membrane may be damaged by coarse contamination, such as dust or other dirt particles in the environmental air, sucked in via the intake tract, for example. This may result in a direct contamination of the sensor element and may destroy it.
Furthermore, a protective device for a moisture sensor is known from DE 10 2006 033 251 A1, which is supposed to protect the sensor from aggressive substances in a fluid that is to be analyzed. The protective device proposed in this document provides a cap that encloses the sensor, which is provided with openings which, in turn, are provided with a membrane that is permeable to water vapor.
In the case of many such sensor devices, especially up-to-date moisture sensors, at least one sensor element is used as a rule. This at least one sensor element, as was also described in the abovementioned related art, is as a rule at least partially protected, by only one protective membrane, from a direct contact with the flow of the fluid medium.